Conventional drilling techniques utilise down-hole drill bits which are conveyed on lengths of drill pipe, which may be rotated from the surface to turn the drill hits, or alternatively a down-hole positive displacement motor may be used to produce the necessary rotary cutting action.
Debris and cuttings are produced in these processes, and this debris is circulated and transported to the surface by the drilling mud or well bore fluid. This fluid is typically pumped from the surface along the centre of the drill pipe, and exits the drill pipe near the milling head into the annulus (i.e. the region within the well bore but external to the drill pipe itself). It will be appreciated that a relatively high pressure is required to maintain this circulation.
It may also be necessary to monitor the axial position and angular orientation of a tool, such as a whipstock, within a well bore, and this is most commonly achieved using a measurement-while-drilling (MWD) tool. A MWD tool requires, however, a relatively high-pressure flow of fluid through the drill pipe to operate.
In many circumstances, a drill string terminates in a hydraulically-set packer, which expands when it is set to press tightly against the inner surface of the bore, thus providing a secure anchoring point for other components further up in the bore. For instance, one or more shear bolts or wires may be broken to activate the packer, or the packer may be activated by inflation.
Hydraulically-set packers are activated automatically when the pressure of fluid at the packer reaches a certain level. If, therefore, large quantities of well fluid are circulated into the drill pipe, as discussed above, the hydraulic packer may accidentally be set prematurely. A bypass valve is therefore required to allow the well fluid to be circulated to the annulus relatively freely before it is desired to set the packer, but to divert well fluid to the packer under high pressure when the packer is to be set.
Alternatively, packers can be set with setting tools, and it has also previously been proposed to use a “drop-ball” system, in which a ball is dropped down the drill pipe to activate a switch, which diverts the well fluid to the packer. These drop-ball systems suffer from certain disadvantages, however, since there is a limit to the number of balls that can be dropped, or in some systems a bore cuts off circulation to the main bore once it has landed on its seat. In addition, it is usually not possible for the ball to pass through a MWD tool, and therefore valves using a drop-ball system can generally only be used higher up in the drill string than an MWD tool. This is undesirable, since the fluid that passes through the MWD tool is preferably circulated to the annulus through the valve, and therefore ideally the valve is placed downstream of the MWD tool.
Other bypass valves utilise indexing tracks, in which a piston moves within a housing, but one or more protrusions on the external surface of the piston follow guide tracks formed on the inside of a housing. A first section of the guide track may be a “zig-zag” shape, so that the piston may be driven up and down a limited distance with respect to the housing and the piston will rotate as it follows the zig-zag track, and during this motion respective apertures formed in the piston and housing are at least partially aligned to allow well fluid to circulate to the annulus. At a certain point in the track, however, the track allows the piston to travel further downwards with respect to the housing, by means of a section of track which extends downward below the main zig-zag pattern, to a position in which the apertures do not align and circulation to the annulus is prevented. Such a system is disclosed, for example, in WO97/21020.
Such systems suffer from disadvantages, however, in that the piston is required to rotate axially within the housing in order to follow the indexing track, and this results in a frictional component from the seals of the valve, which resist this rotational movement, leading to undesirable wear and tear.